Lawn boy silver series pro 6.5 2 cycle

I found a poster with the same identical problem I have, but he never came back and said what the fix was, so I copied it and I'm going to paste it just like he wrote it.

Okay... going to give a good bit of detail here but I'll try to do so w/o writing a novel.

I have a 6.5 HP 2-cycle LawnBoy Silver Series Duraforce self-propelled mower (Model # 10324) bought new 9 years old.

Gave me absolutely no trouble for first 8 years.

Was in shop several months ago for the first time ever... throttle lever control began having absolutely no control on the engine speed but would still allow the mower to be put in choke mode for starting. That's the primary reason I took it in. Also, I noticed about this same time that the "click" or resistance that I'd felt in the throttle control level coming out of "choke mode" and into "full-speed" mode was no longer there. I think this might be referred to "detent", but I'm not sure. Basically, the throttle lever was very fluid all the way from low speed at the bottom all the way up through choke mode and there was no distinct resistance point where you knew/felt that you were about to leave full throttle mode and "click" the remaining way of the control lever into "choke mode". Turned out a throttle/governor (one of those) springs was either off / missing / stretched or something like that. Throttle cable was replaced at the shop, too, for some reason. Exhaust ports were severely gummed-up, too, & were cleaned.

Took mower home & mowed like a dream... for a single mowing. Then, on next attempt at mowing several days later, mower would not crank & would only sputter and made hissing sounds like air was escaping. Throttle lever controlled the engine speed and choke mode properly now, but still no resistance/click point just as I'd described in the previous paragraph.

Back to the shop... shop is a VERY reputable shop in my area, but he's got a preference of NOT working on many Lawn Boys, as he told me they're hard to get into, hard to work on, and that he can typically fix 5 other mowers in the time it takes to fix a single Lawn Boy. Nonetheless, he was that far into repairing my mower the first time and wanted to make sure that he got the thing back to me in good working order, so he continued working on it for me.

Said he checked everything he knew to check and 10-or-so mechanics worked on it over a period of a few weeks and they all were stumped and said there was absolutely no reason why the thing shouldn't run. Said he checked the carb and all was good there. Said that the mower was getting good compression. Said that spark plug was fine. Said that I had no leaking/blown head gaskets. Finally gave up and gave the mower back to me.

So, I took it to an authorized Lawn Boy dealer in town, left it there for a month & finally got it back a week ago.

Oddly enough, they said the problem was a blown top head gasket, which was one of the things that specifically the original mower repair shop said was NOT the problem. Who am I to argue, as long as the 2nd repair shop said they'd identified the problem & repaired it.

They replaced the top head gasket and said that either the throttle or governor spring was "messed up" and they replaced it as well.

Mower cranked on first pull when I got it back and ran like a million bucks for the first mowing.

After the first mowing, I installed an inline fuel shutoff valve as I'd sort of always wanted one on there. Easy enough and did that within about 10 minutes.

Mowed for the 2nd time (yesterday) since I'd gotten it back from the shop and now all of a sudden it's surging. Surges HEAVILY when engine throttle is low, but I of course never mow with it anywhere other than full-throttle as I know that's best. Surges moderately with throttle wide-open. Has never surged like that before. Quits surging for the most part when I'm actually mowing grass, but when stopping to turn or going down the sidewalk or whatever and not actually mowing any grass, it surges and this is bothering the heck out of me.

I tend not to think that the fuel shut-off valve I installed several days ago has anything to do with it, as the valve is fully-open when I'm mowing and there should be full gas flow as usual from the tank to the mower. I have an idea it's something to do with engine rpm setting, governor setting, throttle spring, or some combination of all of those items.

What it does when not cutting grass is technically not of any concern. Specifically, it is running as-designed. The later 2-cycles were forced to run leaner than they should because of emissions laws, so they surge without a load on the engine. When the engine starts to slow because it's cutting, the reduced speed allows the spring-loaded air vane to move, opening the throttle.
Still, you might try increasing governor spring tension to pick up a few RPM which sometimes smooths out the surging and makes it go into 'power mode' (smooth hum) sooner. Find the white plastic wheel with knurled rim at the base of the air vane on top of the carb. Turn the wheel 4 clicks clockwise and see how that works. It is not possible to adjust it enough to over-speed. In fact, as long as the mulch fan and blade are attached, you can manually push the air vane all the way over to wide-open throttle and hold it there. It can only run up so far until the horsepower requirements of the two 'fans' meets the max power of the engine, probably 4000 RPM or so. Too fast to use for mowing, but much slower than the engine can potentially run.

Since the problem seemed to start after installing the fuel valve, I'm thinking a piece of rubber from the inside of the fue line got into the carb. The old fuel lines tend to come apart on the inside. However, it continues to run, so maybe not. Let us know how the speed adjustment works.